I. Problem to Which Invention is Directed.
Polymerization catalysts especially olefin polymerization catalysts (Ziegler) often have a wide particle size distribution (P.S.D.). It has been found that the catalyst particles provide "templates" for the formation of polymer particles; the catalyst particles themselves must be the same shape as the desired final polymer powder shape, desirably with a relatively narrow particle size distribution. Conventional high molecular weight polymer powders made with usual catalysts have a wide P.S.D. This property makes the resulting polymer powder very difficult to handle. A technique that narrows the P.S.D. of catalysts can narrow the P.S.D. of resulting polyolefins and consequently greatly increase their value by virtue of the improved economics of manufacture. Further in order to obtain larger powder particles, the catalyst particles themselves should possess a mean particle diameter of at least 10, preferably at least 20 (most preferably at least 35) microns.
II. Discussion of the Prior Art
The stereospecific polymerization of .alpha.-olefins such as propylene is well known in the art. Polypropylene resin has become a well-established plastic in the world market. Polypropylene powder sales are also increasing rapidly and, at the present time, are increasing more rapidly than sales of polypropylene pellets. The popularity of the polyproplene powder derives, at least in part, from the rapidly expanding use of filled grades of polypropylene, particularly glass- or talc-filled grades.
Most customers demand that polypropylene powder provide all of the resin quality normally obtainable in pelletized products and, in addition, that the powder possess the qualities of good flowability, low fines content, and no "clods" (large aggregates of particles). A reasonably high bulk density is also desirable.
The P.S.D. of catalysts can be made narrower by controlled growth of catalyst particles. Polyolefin P.S.D. can be made narrower by agglomeration and compaction of polymer particles. Adhesive binders have been used in this latter technique; however, adhesives do not function well in catalyst compaction since they poison the catalyst active sites. No prior art is known to the inventor that embodies the concept disclosed here for occluding catalyst fines. Although a Harshaw publication entitled "Harshaw's Dustless Process" describes a similar technique of eliminating dust from inert powders, Harshaw has no contemplation for using the process to obtain catalyst particle agglomerates of better P.S.D. FNT .sup.1 Working directions have been liberally copied herein from that publication.